Optical article having an antistatic layer

ABSTRACT

Disclosed herein is an optical article having a first optical layer; a second optical layer; and an antistatic layer disposed between the first and second optical layers, the antistatic layer having conducting particles having an aspect ratio greater than about 10. The conducting particles may comprise vanadium oxide particles or carbon nanotubes. The optical article may be a brightness enhancement film, a retro-reflecting film, or a reflective polarizer, and be used in a display device, for example, a liquid crystal display device.

FIELD OF THE INVENTION

The invention relates to an optical article having an antistatic layer,and particularly, to an optical article having an antistatic layercomprising conducting particles having a high aspect ratio.

BACKGROUND

Optical articles such as those used in display devices must meetstringent performance criteria including high light transmissivity,clarity, and ultra-clean appearance. Detrimental to optical performanceare defects such as particles, non-planar topography, anddisproportionate degree of contact (sometimes referred to as “wet-out”).These defects can be, in part, a result of static charges that areintroduced by manufacturing, converting, or assembly processes.

For example, static charges can result from a tape (e.g. masking) orother film that is quickly pulled or peeled away from the targetsubstrate/film during processing. These static charges can subsequentlyattract particles of dust or other debris that may be near the surfaceof a film. Particles that eventually land or become anchored on the filmcan lead to unwanted light blockages, refracting, or absorbance,depending on the film's original purpose. A non-planar topography can bethe result of non-uniform shrinkage, warping, or expansion of a film,particularly when an area of the film is pinched or mechanically held inplace while movement or creep occurs with another portion of the film.Another cause, however, may be static charges that can create thepinched or stationary area, causing binding between film layers andconsequently lead to non-uniform or non-synchronized film changes. Theoptical defect known as the “wet-out” phenomenon can occur whendifferences in optical transmission exist between two regions, or wheninterference patterns such as “Newton's rings” are observed. (The defectis minimally detectable when the wet-out is uniform throughout a filmproduct.) Static charges can contribute to non-uniform attraction ofparticular areas between two layered films, causing wet-out.

Accordingly, it is desirable to obtain optical articles having improvedantistatic properties with little or no detrimental effects on opticalperformance.

SUMMARY

Disclosed herein is an optical article comprising a first optical layer;a second optical layer; and an antistatic layer disposed between thefirst and second optical layers, the antistatic layer comprisingconducting particles having an aspect ratio greater than about 10. Theantistatic layer may consist essentially of the conducting particles anda surfactant, such as vanadium oxide particles or carbon nanotubes, anda nonionic surfactant. The optical article may be a brightnessenhancement film, a retro-reflecting film, or a polarizer. The opticalarticle may be used in a display device, for example, a liquid crystaldisplay device.

When conducting particles having an aspect ratio greater than about 10are incorporated into an optical article as described herein, an opticalarticle having a charge decay time of less than about 2 seconds may beobtained with typically little or no detrimental effect on adhesionbetween the first and second optical layers. In addition, the conductingparticles, even though they may be up to about 100 um in at least onedimension, have little or no effect on optical performance of theoptical article.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic cross-section of an exemplary optical article.

FIGS. 2 a-2 c each show a schematic cross-section of an exemplaryoptical article having one or more additional layers.

FIG. 3 shows a schematic cross-section of an exemplary optical articlecomprising a brightness enhancement film.

FIG. 4 shows a schematic cross-section of an exemplary optical articlecomprising a multilayer optical film.

FIG. 5 shows a schematic cross-section of an exemplary display device.

DETAILED DESCRIPTION

Optical articles according to the embodiments described herein canexhibit high resistivity values, yet sustain effective antistaticproperties. For clarity, it is noted that although the term “conductive”is often used in the industry to refer to “static dissipative”, i.e.,antistatic, the terms conductive and antistatic as used herein are notintended to be synonymous. Specifically, a conductive material coatingis considered to have a surface resistivity up to 1×10⁵ ohms/sq, whereasan antistatic material coating typically has a surface resistivity up to1×10¹² ohms/sq. These terms are generally used to describe materialshaving a conductive or antistatic component or agent on an exposedsurface of the material. Optical articles having an antistatic layer“buried” between optical layers having no antistatic properties may bemade such that the optical article is antistatic, even though thearticles exhibit higher levels of surface resistivity. Furthermore, thestatic decay times can be maintained even with these high surfaceresistivity values.

The optical articles disclosed herein are antistatic even in the absenceof circuitry (e.g., wires) connected to the antistatic layer. Exemplaryarticles exhibit sufficient antistatic properties so as to prevent dust,dirt, and other particles from adhering to the surface(s) of the opticalarticle. Surprisingly, the optical articles disclosed herein can exhibita surface resistivity greater than about 1×10⁸ ohms/sq, for example,1×10¹⁰ or greater, yet maintain their antistatic properties. Inaddition, the optical articles disclosed herein may exhibit static decaytimes of less than about 2 seconds.

The optical article disclosed herein comprises conducting particleshaving a high aspect ratio of greater than about 10. This high aspectratio presumably leads to highly efficient formation of antistaticnetworks at extremely low add-on of particles. Under these conditions,in addition to not affecting adhesion between the first and secondoptical layers, this antistatic layer has minimal impact on opticalperformance of the finished article. This is surprising, given that theymay be up to about 100 um in at least one dimension. Typically,immiscible particles of this size incorporated into optical articleshave a detrimental impact on optical performance due to light scatteringarising from refractive index mismatch.

FIG. 1 shows one embodiment in which optical article 10 comprisesantistat layer 12 disposed between and in substantial contact with firstoptical layer 14 and second optical layer 16. The components of theantistat layer may be described as being buried or sandwiched betweentwo optical layers, or on the first optical layer and covered,overcoated, or overlaid with the second optical layer.

The antistatic layer comprises conducting particles having an aspectratio greater than about 10, the aspect ratio determined by dividing thelength of the particles by their diameter. In one embodiment, theconducting particles in the antistatic layer comprise vanadium oxide asdescribed, for example, in U.S. Pat. No. 5,427,835 and references citedtherein. The vanadium oxide particles may be referred to as colloidalparticles, comprising single or mixed valance vanadium oxide such thatthe formal oxidation states of the vanadium ions are typically +4 and+5. Such species may be referred to as V₂O₅. In the aged colloidal form(several hours at 80° C. or more or several days at room temperature),vanadium oxide consists of dispersed fibrillar particles of vanadiumoxide having a width of from about 0.02 to about 0.08 um and a length offrom about 1.0 to about 4.0 um.

The vanadium oxide particles described above may be prepared in the formof colloidal dispersions, sometimes described as sols, as described inU.S. Pat. No. 5,427,835 and references cited therein. One example of asuitable method involves hydrolysis and condensation of vanadiumoxoalkoxides. Vanadium oxoalkoxides may be prepared in situ from aprecursor, such as vanadium oxyhalide or vanadium oxyacetate, and analcohol. Examples of useful precursors include vanadium oxychloride,VOCl₃, and vanadium oxyacetate, VO₂OAc. Suitable alcohols includei-BuOH, i-PrOH, n-PrOH, n-BuOH, and t-BuOH.

The vanadium oxoalkoxide may be a trialkoxide of the formula VO(OR)₃,wherein each R is independently an aliphatic, aryl, heterocyclic, orarylalkyl group, for example, groups such as C₁₋₁₀ alkyls, C₁₋₁₀alkenyls, C₁₋₁₀ alkynyls, C₁₋₁₈ aryls, C₁₋₁₈ arylalkyls, or mixturesthereof, which can be linear, branched, substituted, or unsubstituted,or cyclic. In particular, each R may independently be an unsubstitutedC₁₋₆ alkyl. Heterocylic groups include one or more heteroatoms such asnitrogen, oxygen, or sulfur, for example, furan, thymine, hydantoin, andthiophene. Branched groups include substituents that do not interferewith the desired product, for example, Br, Cl, F, I, OH. Each R groupmay be independently selected, that is, not all R groups in the formulaVO(OR)₃ are required to be the same. An example of a suitable vanadiumoxoalkoxide is vanadium triisobutoxide oxide VO(O-i-Bu)₃. If thevanadium oxoalkoxide is generated in situ, it may be a mixed alkoxidehaving from one to three alkoxide groups. For example, the product ofthe in situ reaction of vanadium oxyacetate with an alcohol is a mixedalkoxide/acetate.

The hydrolysis and condensation reactions used to form the vanadiumoxide particles can be carried out in water within a temperature rangein which the solvent, which preferably is deionized water or a mixtureof deionized water and a water-miscible organic solvent such as a lowmolecular weight ketone or an alcohol, is in a liquid form, e.g., withina range of about 0-100° C. A hydroperoxide such as H₂O₂ or t-butylhydroperoxide may be used to improve dispersability of the particles andfacilitate production of the antistatic layer with highly desirableproperties. When used, the hydroperoxide is typically present in anamount such that the molar ratio of vanadium oxoalkoxide tohydroperoxide is from 1:1 to 4:1. Optionally, reaction can be modifiedby the addition of co-reagents, metal dopants, etc., subsequent aging orheat treatments, or removal of alcohol by-products. Usefulconcentrations of the colloidal dispersions are up to 5 wt % vanadiumoxide particles.

In another embodiment, the conducting particles having an aspect ratioof at least 10 may comprise carbon nanotubes as described, for example,in WO 2004/052559 and references cited therein. The carbon nanotubes maybe single-walled carbon nanotubes which may be viewed as a graphitesheet rolled up into a nanoscale tube. Useful single-walled carbonnanotubes may have a length of 100 um or less and a diameter of 2 nm orless. There may be additional graphene tubes around the core of asingle-walled nanotube which are referred to as multi-walled carbonnanotubes. Useful multi-walled carbon nanotubes may have a length of 100um or less and a diameter of 30 nm or less.

Chemical and physical variations of the carbon nanotubes as described inWO 2004/052559 may be employed in the antistatic layer. They may be inan aggregated form comprising bundles of tubes called ropes, and/oraggregates of ropes called snakes. The nanotubes, ropes, and/or snakesmay form a network with sufficient open area so as not to adverselyeffect optical performance of the optical article. Small diameter ropesthat have not fully integrated/merged into a network may be used aswell. The carbon nanotubes may be oriented in some direction, forexample, in the plane of the antistatic layer, or they may be in arandom arrangement. The carbon nanotubes may also be bent, straight, orsome combination thereof. In addition, they may be modified chemicallyto incorporate chemical agents or compounds, or physically to createeffective and useful molecular orientations, or to adjust the physicalstructure of the nanotube.

A variety of methods are known for preparing single-walled carbonnanotubes. For example, U.S. Pat. No. 5,424,054 describes a method inwhich carbon vapor is produced by electric arc heating of solid carbonwhich is then contacted with a cobalt catalyst; the carbon vapor mayalso be produce by laser heating, electron beam heating, or RF inductionheating. Another method involves the use of a high-temperature laser tosimultaneously vaporize graphite rods and a transition metal asdescribed in Guo, T. et al., Chem. Phys. Lett. 243: 1-12 (1995) andThese, A., et al. Science, 273: 483-487 (1996). Still yet another methodis described in U.S. Pat. No. 6,221,330 in which gaseous carbonfeedstocks and unsupported catalysts are employed.

In general, the amount of conducting particles used in the antistaticlayer will depend upon the particular particles being used, othercomponents in the antistatic layer, the nature of the first and secondoptical layers, etc., as well as on the application in which the opticalarticle is to be used. In most cases, it is desirable to minimize theamount of conducting particles used in order to minimize cost and anyadverse effects on the performance of the optical article. For example,if the conducting particles are capable of imparting color to theoptical article, and the optical article needs to be colorless, then theamount of conducting particles used in the antistatic layer should beminimized to the extent that the optical article remains colorless.Ionic materials such as quaternary ammonium salts are often used ascolorless antistatic agents. However, their antistatic propertiesgenerally depend on absorption of water from their surroundings,resulting in humidity-dependent antistat behavior. In extreme cases,they can cease to function at all as antistats in extremely dryenvironments. Conducting materials are thus preferred in this regard,since their antistatic behavior is independent of ambient humidity. Foranother example, the amount of conducting particles used should notinterfere with adhesion between the first and second optical layers.ASTM D 3359 is a well known method used to measure adhesion between twolayers as described in the examples below, and it is typically desirablefor the adhesion between the two layers to be at least 3.

One way to determine the amount of conducting particles required in theantistat layer is to form the layer on the substrate and then measurethe surface resistivity; ideally the antistat layer has a surfaceresistivity of 1×10¹⁰ ohms/sq or less, for example, about 1×10⁸ ohms/sq.Another useful parameter is charge decay time, that is, the amount oftime it takes for a static charge to decay to 10% its initial value overa given range of voltage, e.g., 5000 V to less than 500 V. For mostcases, the antistat layer has a charge decay time of less than about 2seconds.

The antistatic layer may comprise a variety of components in addition tothe conducting particles having an aspect ratio greater than about 10.In one embodiment, the antistatic layer consists essentially of theconducting particles and a surfactant, e.g., the latter may comprisegreater than about 90 wt % of the total weight of the antistatic layer.The surfactant may be used to aid coatability of the aqueous dispersionwhich is dried to form the antistat layer. The ratio ofsurfactant:antistat in the layer may range from 1:1 to 100:1 by weight.Examples of suitable surfactants include nonionic surfactants such asthe branched secondary alcohol ethoxylates available as Tergitol™surfactants from Dow Chemical Co., and primary alcohol ethoxylates suchas Tomadol® 25-9 from Tomah Chemical Co.

The antistatic layer may comprise a polymeric binder in order tofacilitate coating or reduce streaking or reticulation of the layer upondrying. If used, the binder:antistat ratio is less than about 10:1 byweight. Suitable polymeric binders are materials that are compatiblewith the particles such that stable, smooth solutions or dispersions areformed with little or no agglomeration of the particles. The polymericbinder desirably does not interfere with the antistatic capacity of theparticles. In general, polymeric binders can increase the surfaceresistivity of the resulting antistatic layer, and they may also giverise to hazy coatings due to the high (25-50 wt %) particle loadingsrequired. Haze is known to have an adverse effect on opticalperformance, particularly optical gain. Thus, when used, an antistaticlayer further comprising a polymeric binder preferably has an averagetotal thickness of less than about 25 nm. With or without a polymericbinder, it may also be preferable for the antistatic layer to have anaverage total thickness of less than about 10 nm, with the conductingparticles comprising an average nominal thickness of less than about 2nm.

Typical binders may comprise a condensation or addition polymer, a blendthereof, or a polymer that is some combination thereof. Examples ofcondensation polymers include polyesters, polycarbonates, polyurethanes,polyamides, polyimides, and the like. Examples of addition polymersinclude poly(meth)acrylates, polystyrenes, polyolefins, cyclic olefins,epoxies, polyvinyl chloride, polyvinylidene fluoride, polyethers,cellulose acetates, and the like. The binder may also be crosslinked.

The antistatic coating formulation may be water- or solvent-based,although water-based is preferred because it avoids the need to handleflammable or combustible solvents and avoids emission of volatileorganic compounds to the atmosphere.

The first and second optical layers are able to manage light such thatthe light is intentionally enhanced, manipulated, controlled,maintained, transmitted, reflected, refracted, absorbed, etc. Examplesof optical films include polarizers such as reflective and absorbingpolarizing films, prism films, retro-reflective films, light guides,diffusive films, brightness enhancement films, glare control films,protective films, privacy films, or a combination thereof.

The first and second optical layers may comprise any material suitablefor use in the above-mentioned optical articles and depending on theparticular application or device in which the article will be used.Exemplary properties include optical effectiveness over diverse portionsof the ultraviolet, visible, and infrared regions, optical clarity, highindex of refraction, durability, and environmental stability.Preferably, the first and second optical layers are substantiallyspecular in that they absorb substantially no light over a predeterminedwavelength region of interest; i.e., substantially all light over theregion that falls on the surface of a first or second optical layer isreflected or transmitted. In general, optical layers have a high lighttransmission, for example, greater than about 90%, or greater than about92%. In general, the first and second optical layers may be the same, orthey may be different from each other.

Typically, the optical layers comprise a condensation or additionpolymer, a blend thereof, or a polymer that is some combination thereof.Examples of condensation polymers include polyesters, polycarbonates,cellulose acetate esters, polyurethanes, polyamides, polyimides,poly(meth)acrylates, and the like. Examples of addition polymers includepoly(meth)acrylates, polystyrenes, polyolefins, polypropylene, cyclicolefins, epoxies, polyvinyl chloride, polyvinylidene fluoride,polyethers, cellulose acetates, polyethersulfone, polysulfone,fluorinated ethylenepropylene (FEP), and the like.

In some cases, such as for an optical layer having a microstructuredsurface as described below, the layer may be made by coating a flowablecomposition onto a microstructured tool or liner and then hardening thecomposition. For example, the flowable composition may be radiationcurable and comprise a reactive diluent, oligomer, crosslinker, and anoptional photoinitiator which are hardened or cured by application ofUV, electron beam, or some other kind of radiation after coating ontothe microstructured tool or liner. For another example, the flowablecomposition may be a composition that is made flowable at an elevatedtemperature and then cooled after coating onto the microstructured toolor liner. Examples of useful radiation curable compositions aredescribed below for a microstructured layer.

The first and/or second optical layers may comprise multilayer opticalfilms such as polarizers which are often used in display devices forincreasing the brightness of the display. In general, multilayer opticalfilms such as reflective polarizers comprise hundreds of alternatinglayers of two different polymeric materials. FIG. 4 shows a schematiccross-section of an exemplary optical article 40 comprising firstoptical layer 45, shown as a multilayer optical film having alternatinglayers 42 and 44. Also present is second optical layer 48 and antistaticlayer 46 which is disposed between the first and second optical layers45 and 48.

Materials used in multilayer optical films include crystalline,semi-crystalline, or amorphous polymers such as, for example,PEN/co-PEN, PET/co-PEN, PEN/sPS, PET/sPS, PEN/ESTAR, PET/ESTAR,PEN/EDCEL, PET/EDCEL, PEN/THV, and PEN/co-PET wherein PEN ispolyethylene naphthalate, co-PEN comprises a copolymer or blend basedupon naphthalene dicarboxylic acid, PET comprises polyethyleneterphthalate, sPS comprises syndiotactic polystyrene, and ESTARcomprises a polycyclohexanedimethylene terephthalate from EastmanChemical Co., EDCEL comprises a thermoplastic polymer from EastmanChemical Col, THV is a fluoropolymer from 3M Company, and co-PETcomprises a copolymer or blend based upon terephthalic acid. The entirethickness of the multilayer optical film is desirably from 5 to 2,000μm.

Multilayer optical films are described in U.S. Pat. Nos. 5,882,774;5,828,488; 5,783,120; 6,080,467; 6,368,699 B1; 6,827,886 B2; U.S.2005/0024558 A1; U.S. Pat. Nos. 5,825,543; 5,867,316; or 5,751,388; or5,540,978. Examples include any of the dual brightness enhancement film(DBEF) products or any of the diffusely reflective polarizing film(DRPF) products available from 3M Company under the Vikuiti™ brand,including DBEF-D200 and DBEF-D440 multilayer reflective polarizers.

The relative positions of the antistatic layer and the optical layerscan be such that the components of the antistatic layer are, forexample, buried, sandwiched, covered, overcoated, or overlayed. As such,the optical article may be made using any of several known processessuch as extrusion, coextrusion, coating, and lamination. For example,the antistat layer may be formed by coating the dispersion onto thefirst or second optical layer using techniques such as hand coating (forexample, using a Mayer bar), dip coating, spin coating, roll coating,spray coating, printing, painting, and the like. Once the dispersion isapplied, it can be dried either at room temperature or at an elevatedtemperature up to about 150° C.

The antistat layer may be in substantial contact with the first and/orsecond optical layers. Substantial contact means that no other material(such as air voids) is present between the antistatic layer and theoptical layers. The surfaces of the first and/or second optical layersmay be primed in order to enhance interlayer adhesion in the finalconstruction. Examples of priming include chemical methods such asapplication of polymeric primer coatings, or physical methods such asexposure to corona discharge, plasma, flash lamp, or flame treatment.

The optical article may comprise one or more additional layers on one orboth of the first and second optical layers. In FIG. 2 a, opticalarticle 20 comprises additional layer 22 disposed on first layer 14. InFIG. 2 b, additional layer 22 is disposed on second layer 16. In FIG. 2c, additional layer 22 is disposed on first optical layer 14 and twoadditional layers 22 are disposed on second layer 16. Any number ofadditional layers may be used, and they may all be the same or differentfrom each other, or some may be the same and some different.

The one or more additional layers may be or comprise a diffusive layer,a matte layer, abrasion resistant layer, a layer for chemical or UVprotection, support layer, magnetic shield layer, adhesive layer, primerlayer, skin layer, dichroic polarizer layer, or combinations thereof.Examples of useful support layers include polycarbonate, polyester,acrylic, metal, or glass. The one or more additional layers may beextruded with other layers of the optical article, coated, or laminated.

The optical article disclosed herein may comprise a brightnessenhancement film, such as those used in display devices for increasingthe brightness of the display. These optical articles recycle lightthrough a process of reflection and refraction that ultimately helps todirect light toward a viewer (usually positioned directly in front ofthe display device) that would otherwise leave the screen at a highangle, missing the viewer. A comprehensive discussion of the behavior oflight in a brightness enhancement film may be found, for example, inU.S. Ser. No. 11/283307. Examples include the Vikuiti™ BEFII and BEFIIIfamily of prismatic films available from 3M Company, St. Paul, Minn.,including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, and BEFIIIT.Brightness enhancement films can act as retro-reflecting films orelements for use therewith.

FIG. 3 shows a schematic cross-section of an exemplary brightnessenhancement film 30 comprising first optical layer 32 havingmicrostructured surface 34, antistatic layer 36, and second opticallayer 38. The microstructured surface comprises an array of prisms fordirecting light as described above. The microstructured surface may alsocomprise, for example, a series of shapes including ridges, posts,pyramids, hemispheres and cones, and/or they may be protrusions ordepressions having flat, pointed, truncated, or rounded parts, any ofwhich may have angled or perpendicular sides relative to the plane ofthe surface. Any lenticular microstructure may be useful, for example,the microstructured surface may comprise cube corner elements, eachhaving three mutually substantially perpendicular optical faces thattypically intersect at a single reference point, or apex. Themicrostructured surface may have a regularly repeating pattern, berandom, or a combination thereof. In general, the microstructuredsurface comprises one or more features, each feature having at least twolateral dimensions (i.e. dimensions in the plane of the film) less than2 mm.

Surface 39, opposite the microstructured surface, is generally planarand may be smooth (any structures thereon are small in comparison to thesize of the structures on the microstructured surface) or matte to helphide any structure of a backlight positioned behind the brightnessenhancement film 30, as described below for a display device.

The microstructured layer may be prepared using a polymerizablecomposition, a master having a negative microstructured molding surface,and a preformed second optical layer sometimes referred to as a baselayer. The polymerizable composition is deposited between the master andthe second optical layer, either one of which is flexible, and a bead ofthe composition is moved so that the composition fills themicrostructures of the master. The polymerizable composition ispolymerized to form the layer and is then separated from the master. Themaster can be metallic, such as nickel, nickel-plated copper or brass,or can be a thermoplastic material that is stable under polymerizingconditions and that preferably has a surface energy that permits cleanremoval of the polymerized layer from the master. The microstructuredlayer may have a thickness of from about 10 to about 200 um.

The polymerizable composition may comprise monomers including mono-,di-, or higher functional monomers, and/or oligomers, and preferably,those having a high index of refraction, for example, greater than about1.4 or greater than about 1.5. The monomers and/or oligomers may bepolymerizable using UV radiation. Suitable materials include(meth)acrylates, halogenated derivatives, telechelic derivatives, andthe like, and as described in U.S. Pat. Nos. 4,568,445; 4,721,377;4,812,032; 5,424,339; and U.S. Pat. No. 6,355,754; all incorporated byreference herein. A preferable polymerizable composition is described inU.S. Ser. No. 10/747985, filed on Dec. 30, 2003, and which isincorporated herein by reference. This polymerizable compositioncomprises a first monomer comprising a major portion of 2-propenoicacid, (1-methylethylidene)bis[(2,6-dibromo-4,1-phenylene)oxy(2-hydroxy-3,1-propanediyl)]ester;pentaerythritol tri(meth)acrylate; and phenoxyethyl(meth)acrylate.

The particular choice of materials used for the polymerizablecomposition will depend upon the method used to form the microstructuredlayer, for example, viscosity may be an important factor. The particularapplication in which the brightness enhancement film will be employedmay also be considered, for example, the film needs to have particularoptical properties yet be physically and chemically durable over time.

The second optical layer in a brightness enhancement film may bedescribed as a base layer. This layer may comprise any material suitablefor use in an optical product, i.e., one that is optically clear anddesigned to control the flow of light. Depending on the particularapplication, the second optical layer may need to be structurally strongenough so that the brightness enhancement film may be assembled into anoptical device. Preferably, the second optical layer adheres well to thefirst optical layer and is sufficiently resistant to temperature andaging such that performance of the optical device is not compromisedover time. Materials useful for the second optical layer includepolyesters such as polyethylene terephthalate, polyethylene naphthalate,copolyesters or polyester blends based on naphthalene dicarboxylicacids; polycarbonates; polystyrenes; styrene-acrylonitriles; celluloseacetates; polyether sulfones; poly(methyl)acrylates such aspolymethylmethacrylate; polyurethanes; polyvinyl chloride;polycyclo-olefins; polyimides; glass; or combinations or blends thereof.The second optical layer may also comprise a multilayered optical filmas described above and in U.S. Pat. No. 6,111,696 which is incorporatedherein by reference.

FIG. 5 shows a schematic cross-section of an exemplary display device 50comprising the optical article 52 disclosed herein. In general, lightsource 54 emits light, depicted by rays 56, that propagates throughoptical article 52 and illuminates display panel 58 making an image orgraphic visible for one or more viewers 60 disposed on the opposite sidethereof.

Display panel 58 may comprise any type of display that is capable ofproducing images, graphics, text, etc. In some display devices, images,graphics, text, etc. may be produced from an array of typicallythousands or millions of individual picture elements (pixels) that maysubstantially fill the lateral extent (length and width) of the displaypanel. The array of pixels may be organized in groups of multicoloredpixels (such as red/green/blue pixels, red/green/blue/white pixels, andthe like) so that the displayed image is polychromatic. The pixels mayalso be such that the displayed image is monochromatic. In oneembodiment, display panel 58 is an LCD panel which typically comprises alayer of liquid crystalline material disposed between two glass plates,and a controller is used to activate selectively the pixels such thatthe images, graphics, text, etc. are viewable on the side of the displaypanel opposite light source 54.

In general, light source 54 may comprise any type and/or configurationof light source typically used in display devices. For example, lightsource 54 may comprise one or more cold cathode fluorescent lamps(CCFLs), hot cathode fluorescent lamps, incandescent lamps,electroluminescent lights, phosphorescent lights, light emitting diodes(LEDs), or combinations thereof. Light emitted by light source 54 may bewhite, red, green, or blue, for example, or some combination thereof.

Light source 54 may be disposed directly behind the display device inwhat is known as a direct-lit configuration. Alternatively, an edge-litconfiguration may be used wherein the light source is disposed along anedge of the display device with a light guide positioned to guide lightto directly behind the display panel. When one or more light sources isused, they may be disposed in rows, e.g., along reflective strips ofmaterial, or they may be disposed in rings, modules, hexagonal latticearrays, at random, or some combination thereof. In some cases, the lightsource may comprise one or more LEDs, such as an array of twenty orhundreds of LEDs. In any case, the number of light sources, the spacingbetween them, and their placement relative to other components indisplay device 50 can be selected as desired depending on designcriteria such as power budget, thermal considerations, size constraints,cost, and so forth.

Back reflector 62 is disposed behind light source 54 to form a lightrecycling cavity within which light can undergo successive reflectionsuntil it is able to propagate towards the display panel. For optimumillumination and efficiency, it is typically advantageous for backreflector 62 to have overall high reflectivity and low absorption. Backreflector 62 may be a specular reflector, for example, the multilayerpolymeric films available as Vikuiti™ ESR from 3M Company, and aluminumreflector sheets such as MIRO® products available from AlanodAluminum-Veredlung GmbH & Co. Back reflector 62 may be a diffusereflector comprising a pressed cake or tile of a white inorganiccompound, or a polymeric article loaded with diffusely reflectiveparticles, air-filled voids, or having polymer domains formed bythermally induced phase separation.

Examples of display devices include laptop computers, computer monitors,hand-held devices such as cell phones and calculators, digital watches,televisions, and the like.

EXAMPLES Example 1

A vanadium oxide colloidal dispersion was prepared as described in U.S.Pat. No. 5,427,835 and diluted with deionized water to give 0.01 wt %concentration. Tergitol® TMN-6 (surfactant from Dow Chemical) was addedat 0.1 wt. % to aid coatability. The dispersion was then coated using aNo. 3 wire-wound rod on primed 5 mil PET film (prepared according toExample 29 of U.S. Pat. No. 6,893,731 B2). The coated film was thendried in an oven at 100° C. for 3 minutes and cooled to room temperatureto give a total final dry vanadia coating thickness of 0.2 nm. Surfaceresistivity was then measured using an Electro-Tech Systems, Inc. Model880 Autoranging Resistance Indicator which outputs values in decades.

A radiation-curable composition as described in U.S. Ser. No. 10/747985was prepared by combining a first monomer comprising a major portion of2-propenoic acid,(1-methylethylidene)bis[(2,6-dibromo-4,1-phenylene)oxy(2-hydroxy-3,1-propanediyl)]ester;pentaerythritol tri(meth)acrylate; and phenoxyethyl(meth)acrylate. Thiscomposition was coated over the vanadium oxide layer and cured using aUV Fusion Lighthammer equipped with a D bulb and operating at 100% powerand 20 ft/min line speed under nitrogen purging.

The following procedure was used:

-   -   Heat the resin at 60° C. for 1 hr until liquefied.    -   Heat an unpatterned stainless steel BEF tool on a hot plate at        150° C.    -   Heat a PL1200 laminator to ˜70° C. and set speed to 12 in/min        (setting #2.)    -   Apply a bead line of BEF resin to the tool.    -   Using a hand roller, gently place the coated side of the PET        film against the tool and roll to tack in place.    -   Sandwich the tool+film sample between two larger pieces of        unprimed PET film to protect the laminator rolls.    -   Run sample through the laminator. This gives a total resin film        thickness of 0.4 mil.    -   Pass sample twice through the UV processor.    -   Gently remove film sample from tool.

Charge decay time was measured using an Electro-Tech Systems, Inc. Model406C static decay meter by charging the sample to 5 kV and measuring thetime required for the static charge to decay to 10% of its initialvalue. Adhesion of the cured layer to the PET film was measuredaccording to ASTM D 3359, a crosshatch tape pull test using 3M™ 610cellophane tape (available from 3M Company). Ratings were on a scale of0-5 with 5 being perfect adhesion and 0 being complete delamination.Results are shown in Table 2.

Example 2

Example 2 was prepared and evaluated as described in Example 1 exceptthat the vanadium oxide had a thickness of 0.5 nm.

Comparative Examples 1-12

Comparative Examples 1-12 were prepared and evaluated as described inExample 1 except that the commercially available dispersions listed inTable 1 were used instead of the vanadium oxide dispersion. Thedispersions were used as received from the supplier and coated to givedry thicknesses as shown in Table 2.

Control Example

The Control Example was prepared as described in Example 1 except thatthe vanadium oxide dispersion was not used, that is, theradiation-curable composition was coated directly onto the primedsurface of the PET film.

TABLE 1 Commercial Product Density¹ Conc. Colloid Designation Supplier(g/cc) (wt %) SnO₂ (NH₄ ⁺) SN15CG Nyacol 6.95 15 SnO₂ (K⁺) SN15 Nyacol6.95 15 AZO Celnax ® Nissan 5.5² 30 ZX-330HF2 Chemical ATO StanostatKeeling & 6² 19.1 CPM10C Walker ITO blue ITO Sol, Blue Advanced 6.95² 20Nano Products ITO yellow ITO Sol, Yellow Advanced 6.95² 20 Nano ProductsATO 100 ATO Sol, 100 nm Advanced 6² 20 Nano Products ATO 30 ATO Sol, 100nm Advanced 6² 20 Nano Products ¹from the Handbook of Chemistry andPhysics ²approximate average of the values for the single oxides

TABLE 2 Antistat Layer Est. Ctg. Surface Charge Decay Nominal ThicknessResistivity Time After Resin Resin Example (nm) Colloid (Ω/□) Coating(sec) Adhesion 1 0.2¹ Vanadia 10⁸ 1.65 5 2 0.5¹ Vanadia 10⁸ 0.02 5 Comp.1 12 SnO₂ (NH₄ ⁺)  10¹¹ >30 2 Comp. 2 25 SnO₂ (NH₄ ⁺)  10¹⁰ 5.25 4 Comp.3 12 SnO₂ (K⁺) 10⁹-10¹⁰ 8.15 0 Comp. 4 25 SnO₂ (K⁺) 10⁹ 7.4 0 Comp. 5 16AZO  10¹⁰ >30 5 Comp. 6 31 AZO 10⁹ 0.01 3 Comp. 7 9 ATO  10¹⁰ 0.07 5Comp. 8 17 ATO 10⁸ 0.01 5 Comp. 9 12 ITO blue >10¹² NM NM Comp. 10 12ITO yellow >10¹² NM NM Comp. 11 9 ATO 100 >10¹² NM NM Comp. 12 7 ATO 30>10¹² NM NM Control None none >10¹² >30 5 ¹density of the vanadium oxide= 3.36 g/cc, Handbook of Chemistry and Physics NM = not measured

Example 3

A 1 wt % solution of Triton X-100 surfactant (available from UnionCarbide Corp., a subsidiary of Dow Chemical, Danbury, Conn.) wasprepared by mixing and stirring 0.32 g surfactant and 31.68 g deionizedwater in a glass bottle. To this solution was then added 0.32 gmultiwall carbon nanotubes (available from NanoLab Inc., Newton, Mass.)powder, and the mixture was ultrasonicated for 32 hr. The sonicatedmixture was homogenized using a HandiShear homogenizer (VirTis,Gardiner, N.Y.) at rates ranging from 5000 to 30000 rpm for about twominutes. The resulting dispersion was then poured into a 45 mlcentrifuge tube (VWR, Bristol, Conn.), sealed with a cap, andcentrifuged for 5 min at 3600 rpm in an IEC Centra CL2 benchtopcentrifuge (Thermal Electron Corp., Waltham, Mass.). The dispersion wasdecanted into a clean glass bottle, leaving about 8.81 g residue behindin the centrifuge tube. Thermal and gravimetric analysis of the residueafter drying allowed the concentration of multiwall carbon nanotubes inthe final dispersion to be estimated at 0.35 wt %.

The above dispersion of multi-walled nanotubes (density of thenanotubes=1.4 g/cc)was coated on primed PET film using a #3 wire-woundrod, and the coating was dried in a forced-air oven at 100° C. for 3min. Surface resistivity on the dried coating was measured at 10⁸ohm/sq. Estimated nominal thickness of the carbon nanotubes coating was17 nm. This sample was overcoated with UV-curable resin using thematerial and procedure described in Example 1. Static charge decay timeof the resulting construction was 0.02 sec, and resin adhesion was rated5.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof the invention, and it should be understood that this invention is notlimited to the examples and embodiments described herein.

1. An optical article comprising: a first optical layer; a secondoptical layer; and an antistatic layer disposed between the first andsecond optical layers, the antistatic layer comprising conductingparticles having an aspect ratio greater than about 10 and comprisingsingle-walled carbon nanotubes having a length of 100 um or less and adiameter of 2 nm or less, wherein the antistatic layer has an averagetotal thickness of less than about 10 nm and exhibits a surfaceresistivity of greater than about 1×10⁸ ohms/sq.
 2. The optical articleof claim 1 having a charge decay time of less than about 2 seconds,wherein charge decay time is measured by the time required for staticcharge of the article to decay to 10% of its initial value.
 3. Theoptical article of claim 1, the first and second optical layers havingan adhesion of at least 3 according to ASTM D
 3359. 4. The opticalarticle of claim 1, at least one of the first and second optical layerscomprising a microstructured surface.
 5. The optical article of claim 1,at least one of the first and second optical layers comprising amultilayer optical film, the multilayer optical film comprisingalternating layers of at least two different polymeric materials.
 6. Theoptical article of claim 1, the antistatic layer in substantial contactwith the first and/or second optical layers.
 7. A display devicecomprising: a light source; a display panel; and the optical article ofclaim 1 disposed between the light source and the display panel.
 8. Thedisplay device of claim 7, the display panel comprising a liquid crystaldisplay panel.
 9. The optical article of claim 1, the antistatic layerconsisting essentially of the conducting particles and a surfactant. 10.The optical article of claim 9, the surfactant comprising a nonionicsurfactant.
 11. An optical article comprising: a first optical layer; asecond optical layer; and an antistatic layer disposed between the firstand second optical layers, the antistatic layer comprising conductingparticles having an aspect ratio greater than about 10 and comprisingmulti-walled carbon nanotubes having a length of 100 um or less and adiameter of 30 nm or less, wherein the antistatic layer has an averagetotal thickness of less than about 10 nm and exhibits a surfaceresistivity of greater than about 1×10⁸ ohms/sq.
 12. The optical articleof claim 11 having a charge decay time of less than about 2 seconds,wherein charge decay time is measured by the time required for staticcharge of the article to decay to 10% of its initial value.
 13. Theoptical article of claim 11, the first and second optical layers havingan adhesion of at least 3 according to ASTM D
 3359. 14. The opticalarticle of claim 11, at least one of the first and second optical layerscomprising a microstructured surface.
 15. The optical article of claim11, at least one of the first and second optical layers comprising amultilayer optical film, the multilayer optical film comprisingalternating layers of at least two different polymeric materials. 16.The optical article of claim 11, the antistatic layer in substantialcontact with the first and/or second optical layers.
 17. A displaydevice comprising: a light source; a display panel; and the opticalarticle of claim 11 disposed between the light source and the displaypanel.
 18. The display device of claim 17, the display panel comprisinga liquid crystal display panel.
 19. The optical article of claim 11, theantistatic layer consisting essentially of the conducting particles anda surfactant.
 20. The optical article of claim 19, the surfactantcomprising a nonionic surfactant.